Optical frequency division multiplexing network

ABSTRACT

An optical frequency division multiplexing network includes first optical communication paths connected to terminals, respectively, a second optical communication path connected to the outside and a node composed of a selection unit for selecting signals having optical frequencies to be sent to the plurality of terminals, respectively, from signals transmitted through the second optical communication path in optical frequency division multiplexing, a conversion unit for converting the selected signals into signals having a single optical frequency and an output unit for producing the converted signals to the terminals through the first optical communication paths, respectively.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 09/121,591 filedJul. 24, 1998, now U.S. Pat. No. 6,619,865 B1, which is a continuationof U.S. Ser. No. 08/608,725 filed Feb. 29, 1996, now U.S. Pat. No.5,801,864, which is a continuation of U.S. Ser. No. 08/233,974 filedApr. 28, 1994, now U.S. Pat. No. 5,510,921, which is a continuation ofU.S. Ser. No. 07/800,255 filed Nov. 29, 1991, now U.S. Pat. No.5,321,540.

BACKGROUND OF THE INVENTION

The present invention relates to an information transmission systememploying optical communication, and more particularly to a network withhigh reliability and flexibility using optical frequency selection andoptical frequency conversion functions.

Recently, with the advance of coherent communication techniques, therehas been proposed a network utilizing optical frequency divisionmultiplexing (or optical wavelength division multiplexing) transmission.

Typical examples of the optical frequency or wavelength divisionmultiplexing network are found in paper (1) “IEEE Journal of LightwaveTechnology, Vol. 7, No. 11, pp. 1759–1768, 1989” and paper (2)“Proceedings of IOOC, '90, pp. 84–95, 1990”. Networks described in otherpapers are similar to those described in the above two papers.

A network configuration described in the paper (1) is shown in FIG. 2 ofthe paper and part thereof corresponding to the present invention isshown in FIG. 2 of the accompanying drawings. FIG. 2 shows a linedistribution and collection system of the network shown in the paper(1). The system of FIG. 2 includes a remote node 10 having a wavelengthdemultiplexer 500 and a wavelength multiplexer 501 connected throughoptical fibers 100 and 200, respectively, to a central office andsubscriber terminals 20-1˜N connected through optical fibers 300-1˜N to400-1˜N to the remote node. Signals having wavelength λ₁₁ to λ_(1n)transmitted from the central office in wavelength division multiplexingfashion are demultiplexed into signals having the respective opticalfrequencies by the wavelength demultiplexer to be transmitted to thesubscriber terminals 20-1˜N. On the countrary, signals having wavelengthλ₂₁ to λ_(2n) transmitted from the subscriber terminals 20-1˜N arewavelength-multiplexed by the wavelength multiplexer to be transmittedto the central office.

In the above-mentioned system, the subscriber terminals 20-1˜N musttransmit and receive signals having different wavelengths, respectively.In the paper (1), as shown in FIG. 4 thereof, receivers are common tothe subscriber terminals, while transmitters employ lasers havingdifferent wavelengths for each subscriber terminal. Accordingly, a laserhaving stable wavelength must be provided in each subscriber terminaland hence there is a problem in reliability and flexibility. Further,movement of the subscriber terminal is not easy.

In the paper (1), transmission employs the conventional intensitymodulation optical communication and accordingly it is difficult thatthe multiplex degree of optical signal exceeds 100. Even in this system,a coherent receiver capable of effecting multiplexing with the multiplexdegree of 1000 or more can be used. In this case, receivers capable ofreceiving signals having wavelengths λ₁₁ to λ_(1n) transmitted from thecentral office assigned to the subscriber terminals 20-1˜N withwavelength division multiplexing are required. Accordingly, thereceivers are expensive as compared with the present invention describedlater.

Further, coherent receivers having variable transmission wavelength andcommon to the subscriber terminals 20-1˜N can be employed. In this case,however, signals having wavelength λ₂₁ to λ_(2n) transmitted from thesubscriber terminals are also multiplexed and accordingly the wavelengthmust be stable. It is difficult to remotely control the wavelength andhence the reliability of the network is also degraded.

Furthermore, when it is to be attempted that the optical fibers 300-1˜Nand 400-1˜N are combined to effect bi-directional transmission by meansof a single optical fiber per subscriber terminal, “it is basicallyrequired that all of wavelengths λ₁₁ to λ_(1n) and λ₂₁ to λ_(2n) aredifferent” and utilization efficiency of frequency is deteriorated.

A network configuration described in the paer (2) is shown in FIG. 1 ofthe paper and is shown in FIG. 3 of the accompanying drawings incorresponding manner to the present invention. The system includes aremote node (not shown in the paper (2)) having a power divider 502 anda transport star coupler or wavelength multiplexer 501 connected to acentral office (not shown in the paper (2)) through optical fibers 100and 200 and fixed wavelength receivers and tunable transmitters orsub-scriber terminals 20-1˜N connected to the remote node throughoptical fibers 300-1˜N and 400-1˜N. All optical signals havingwavelengths λ₁₁ to λ_(1n) transmitted from the central office withwavelength division multiplexing are transmitted to the subscriberterminals 20-1˜N by means of the power divider and the subscriberterminals 20-1˜N receive only necessary signals by receivers forreceiving only particular wavelength. On the contrary, signal havingwavelength λ₂₁ to λ_(2n) transmitted from the subscriber terminals arewavelength-multiplexed by the wavelength multiplexer to be transmittedto the central office.

This system is featured in that an inexpensive power divider is usedinstead of the wavelength de-multiplexer of the paper (1) and wavelengthselection reception which is a maximum advantage of coherenttransmission can be utilized.

The maximum drawback of this system is that all of the subscriberterminals 20-1˜N can receive all signals. Thus, there is a problem inprivacy characteristic.

Accordingly, in the system of the paper (2), receivers having fixedreceive frequency are disposed in each of the subscriber terminals20-1˜N. However, there remains the problem in the privacy characteristicfor malicious operation.

Further, when coherent transmitter and receiver are used, thetransmitter and receiver of the system have also the same problem as inthe transmitter and receiver of the paper (1).

The conventional network utilizing the wave-length division multiplexinghas drawbacks as follows. Particularly, since the wavelength employedbetween the central office and the remote node and between the remotenode and the subscriber terminals is the same, a failure occurring inone subscriber terminal influences all of the subscriber terminalsconnected to the remote node to which the subscriber terminal having thefailure is connected. Further, since the transmitter and receiver of thesubscriber terminal must deal with a multiplicity of frequencies andrequire the same reliability as that of the central office, it is veryexpensive. In addition, expansion of the network and rearrangement ofthe subscriber terminals are not made easily and the flexibility of thenetwork is lacking.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a network havingtransmitters and receivers for terminals utilizing inexpensive commonoptical frequency division multiplexing and having good privacycharacteristic, high reliability and flexibility.

In order to achieve the above object, the present invention has thefollowing measures.

-   1. A node for distributing signals transmitted in optical frequency    division multiplexing to terminals selects an optical frequency    corresponding to the terminal from the transmitted signals and    converts the selected optical frequency into an optical frequency    determined in an interface common to the terminals to be transmitted    to the terminals.-   2. A node for collecting signals transmitted from the terminals and    transmitting the signals in optical frequency division multiplexing    fashion converts the signals transmitted with the optical frequency    determined in the interface common to the terminals into optical    frequencies to be transmitted in the optical frequency division    multiplexing fashion.

FIG. 1 shows a basic logical configuration of the present invention. Itcomprises a remote node 10 connected through optical speech paths oroptical channels 100 and 200 to an upper node and terminals 20-1˜Nconnected to the remote node 10 through optical fibers 300-1˜N and400-1˜N. The remote node 10 includes optical frequency selectors 600-1˜Nfor selecting optical frequencies in accordance with control signals650-1˜N, optical frequency converters 601-1˜N for converting opticalfrequency in accordance with the control signals 650-1˜N, opticalfrequency converters 602-1˜N for converting optical frequency inaccordance with control signals 660-1˜N, and a control unit 11 forproducing the control signals 650-1˜N and 660-1˜N. The optical frequencyselectors 600-1˜N select signals having optical frequencies λ₁₁ toλ_(1n) corresponding to the terminals from signals having opticalfrequencies λ₁₁ to λ_(1n) transmitted from the upper node through theoptical channel 100 in the optical frequency division multiplexing inaccordance with the control signals 650-1˜N produced by the control unit11 and the selected signals are converted into signals having opticalfrequency λ₁₀ determined in an interface common to the terminal by theoptical frequency converters 601-1˜N in accordance with the controlsignals 650-1˜N of the control unit 11 to transmit the converted signalsto the terminals 20-1˜N through the optical fibers 300-1˜N. On thecontrary, signals transmitted from the terminals 20-1˜N through theoptical fibers 300-1˜N and having optical frequency λ₂₀ determined inthe interface common to the terminals are converted by the opticalfrequency converters 602-1˜N into signals having optical frequencies λ₂₁to λ_(2n) in accordance with the control signals 660-1˜N of the controlunit 11 and are optical frequency division multiplexed to be transmittedto the upper node.

FIG. 1 shows the logical configuration, while even if the opticalfrequency selection and the optical frequency conversion are replacedwith each other, it can be configured by a functioning portion whichperforms the optical frequency selection and the optical frequencyconversion simultaneously.

Further, the optical frequency of the signals between the terminals andthe node is not limited to one kind, and a system in which the opticalfrequency is selected from predetermined frequencies can be configured.

Transmission between the terminals and the node can be made by theoptical frequency division multiplexing transmission and further by theoptical frequency division multiplexing bi-directional transmission. Atthis time, a plurality of optical frequencies between the terminals andthe node common to the terminals are required.

According to the present invention, since the signal having thefrequency corresponding to the terminal is selected by the opticalfrequency selector and only the signal is optical frequency divisionmultiplexed to be transmitted to the terminal, the privacy is ensured.

Further, since the optical frequencies for communication between theupper node and the remote node and between the remote node and theterminals are assigned independently and are controlled by the controlunit of the remote node, the reliability is high and the flexibility isincreased. In addition, by assigning the optical frequencies between theupper nodes and the remote node dynamically, the high reliable andflexible network can be realized.

The transmit and receive optical frequency of the terminal is common tothe terminals and fixed, and the frequency range is narrow. Even when aplurality of optical frequency are assigned, frequency spacing may bemade wide and accordingly inexpensive and reliable terminals can beattained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a basic logical configuration of thepresent invention.

FIGS. 2 and 3 schematically illustrate prior art configurations.

FIG. 4 schematically illustrates the whole configuration according to anembodiment of the present invention.

FIG. 5 schematically illustrates a configuration of an interface of anupper node.

FIG. 6 schematically illustrates a configuration of an optical frequencyconverter.

FIG. 7 schematically illustrates a configuration of an interface of aterminal.

FIGS. 8A, 8B and 9 schematically illustrate terminal networks.

FIG. 10 schematically illustrates a configuration of an opticalfrequency conversion circuit group.

FIGS. 11A and 11B schematically illustrate configurations of an opticalfrequency conversion circuit.

FIG. 12 schematically illustrates a configuration of an opticalfrequency conversion element.

FIG. 13 schematically illustrates a configuration of a variablewavelength optical source of the optical conversion element.

FIG. 14 schematically illustrates a configuration of a terminalcorresponding interface.

FIG. 15 schematically illustrates an optical signal distribution andcollection portion.

FIGS. 16A, 16B, 17A, 17B, 18A and 18B schematically illustrateconfigurations of terminal nodes.

FIGS. 19A to 19C schematically illustrate configurations of an opticalfrequency demultiplexer of a node.

FIG. 20 schematically illustrates a configuration of an optical signalmultiplexer of a node.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment to which the present invention is applied is now describedwith reference to FIGS. 4 to 20. The embodiment shows one configuration,while actually constituent elements can be omitted or combined dependingon information content and the number of terminals.

FIG. 4 schematically illustrates a configuration of a network of theembodiment. The network comprises a node 10 for distributing informationto terminals, optical fibers 100-01˜B, 100-11˜R, 120-1˜D and 200-11˜Tfor connecting between the node and an upper node, control signal lines190 and 290 for transmitting control information between the node 10 andthe upper node, terminal networks 20-01˜U and 20-11˜F, and opticalfibers 300-01˜U, 400-01˜U and 340-11˜F for connecting between the nodeand the terminal networks. The node 10 includes an upper node signalinterface unit 12, an optical frequency conversion unit 13, a terminalinterface unit 14 and a control unit 11. Signals having frequenciesλ_(5n) , λ_(1n) and λ_(3k) transmitted through the optical fibers100-01˜B, 100-11˜R and 120-1˜D from the upper node are optical frequencydivision demultiplexed or divided in the upper node signal interfaceunit 12 if necessary and are supplied to the optical frequencyconversion unit 13. The signals are further optical frequency convertedby the optical frequency conversion unit 13 selectively in accordancewith a command signal 650 produced by the control unit 11 if necessaryand are multiplexed to signals having frequencies λ_(ou) and λ_(og)corresponding to the terminal networks in accordance with a commandsignal 695 from the control unit 11 by the terminal interface unit 14 ifnecessary to be distributed to the terminal networks 20-01˜U and 20-11˜Fthrough the optical fibers 300-01˜U and 340-11˜F. On the contrary,signals having frequencies λ_(ov) and λ_(of) transmitted from theterminal networks 20-01˜U and 20-11˜F through the optical fibers400-01˜U and 340-11˜F are optical frequency divisiondemultiplexed/multiplexed or divided/optical frequency divisionmultiplexed by the terminal interface unit 14 if necessary and aresupplied to the optical frequency conversion unit 13. Further, thesignals are optical frequency converted by the optical frequencyconversion unit 13 selectively in accordance with the command signal 650from the control unit 11 and are multiplexed by the upper node signalinterface unit 12 if necessary to be transmitted as signals havingfrequencies λ_(4m) and λ_(2n) to the upper node through the opticalfibers 200-11˜T and 120-1˜D. It is assumed that each one of the terminalnetworks 20-01˜U and 20-11˜F corresponds to each one of the subscribersas a rule and the privacy in the network of the embodiment is insuredfor the terminal networks.

Signals are transmitted in the optical frequency division multiplexingfashion from the upper node to the node 10 through the optical fibers100-01˜B and 100-11˜R, from the node 10 to the upper node through theoptical fibers 200-11˜T, and bi-directionally between the upper node andthe node 10 through the optical fibers 120-1˜D. Assignment of theoptical fibers and the optical frequencies to signals is made so thatservice and maintenance are optimum. In the embodiment, broad-castingsignal such as a TV signal is transmitted through the optical fibers100-01˜B. Part of up and down signals of the terminals has the sameoptical frequency in two corresponding optical fibers 100-1i and 200-1i(iε{1 . . . R=T}) on condition that the number of the optical fibers100-11˜R is equal to the number of the optical fibers 200-11˜T (R=T).Further, each one frequency of the up signal frequencies {λ_(4m)} andthe down signal frequencies {λ_(3k)} in one optical fiber of the fibers120-1˜D is assigned to the remaining of the up and down signals of theterminals. Assignment of signals to the optical fiber and the opticalfrequency of the assigned fiber is determined by the upper node, thenode 10 or both of them. In the embodiment, the upper node has the rightof decision and the node 10 performs monitoring/detection of a failureor the like to transmit control information to the upper node throughthe control signal line 290 properly. The upper node assigns the fibersand the optical frequencies to the signals in accordance with lineassignment request from terminal and to terminal, maintenanceinformation, control signal of the node 10 and the like and transmitsthe signals to the node 10 through the control signal line 190. Theassignment involves fixed and semi-fixed assignment (re-assignment ismade only when a failure occurs) and dynamic assignment selected inaccordance with a kind of terminal or the like. Further, there is a casewhere a signal transmitted to one terminal is transmitted to the nodethrough a different fiber. The fibers and the optical frequencies areconfigured redundantly and the fibers and the optical frequencies arere-assigned upon occurrence of a failure.

The optical frequency of the signal between the upper node and the node10 is determined by a kind of signal (analog signal or digital signal),a modulation method, a signal band and an optical circuit component suchas an optical frequency conversion element, while it is set to highdensity. In the embodiment, 32 channels of digital signal having 622Mb/s at its maximum are assigned to bands having optical wavelengths of1.3 μm and 1.5 μm for the optical fibers 100-01˜B at intervals of 10GHz, 128 channels of digital signal having 155 Mb/s at its maximum areassigned to bands having optical wavelengths of 1.3 μm and 1.5 μm forthe optical fibers 100-11˜R and 200-11˜T at intervals of 2.5 GHz, and128 channels of digital signals having 155 Mb/s at its maximum areassigned to bands having optical wavelengths of 1.3 μm for the up signaland 1.55 μm for the down signal for the optical fibers 120-1˜D atintervals of 2.5 GHz.

One optical frequency transmission or optical frequency divisionmultiplexing transmission is made from the node to the terminal networkthrough the optical fibers 300-01˜U, from the terminal network to thenode through the optical fibers 400-01˜U and bi-directionally betweenthe node and the terminal network through the optical fibers 340-11˜F.The optical fibers 300-0i and 400-0i (i=1 . . . U) are wired by two-wirefiber cable.

The optical frequency of the signal between the terminal networks andthe node 10 is determined by a kind of signal (analog signal or digitalsignal), a modulation method, a signal band and an optical circuitcomponent such as an optical frequency conversion element in the samemanner as between the upper node and the node 10, while it is determinedin consideration of conditions on the side of terminal such as a costand a size. In the embodiment, 16 channels of digital signal having 622Mb/s at its maximum are assigned to bands having optical wavelengths of1.3 μm and 1.55 μm for the optical fibers 300-01˜U and 400-01˜U atintervals of 10 GHz, 3 channels are assigned at intervals of 160 GHzfrom the frequency separated from the above frequency by 160 GHz, 16channels of digital signal having 622 Mb/s at its maximum are assignedto bands having optical wavelength of 1.3 μm for the up signal and 1.5μm for the down signal for the optical fibers 340-11˜F at intervals of10 GHz, and 3 channels are assigned at intervals of 160 GHz from thefrequency separated from the above frequency by 160 GHz. The formeroptical frequency having the interval of 10 GHz is assumed to be abroadcasting signal such as a TV signal. One channel of the latter threechannels is for terminal and the remaining two channels are forexpansion.

FIG. 5 schematically illustrates a configuration of the upper nodeinterface unit 12. The upper node interface unit 12 comprises amultiplexer 506 for the down signals having a frequency of λ_(2n)including optical multiplexers 511-1˜D for multiplexing the up signalshaving frequencies λ_(4m) and λ_(2i) transmitted through opticalwaveguides 202-21˜2D from the optical frequency conversion unit 13 tosend the multiplexed signals to a bi-directionalmultiplexing/demultiplexing unit 505 and optical multiplexers 512-1˜Tfor multiplexing the up signals having frequencies λ_(4m) and λ_(2i)transmitted through optical waveguides 202-11˜T from the opticalfrequency conversion unit 13 to produce the multiplexed signals tooptical waveguides 200-11˜T and a bi-directionalmultiplexing/demultiplexing unit 505 including a bi-directionalmultiplexer/demultiplexers or a bi-directional multiplexer/dividers510-1˜D for multiplexing/demultiplexing or multiplexing/dividing thedown signals having frequency of λ_(3k) of bi-directional signals on theoptical waveguides 120-1˜D to be transmitted through the opticalwaveguides 102-21˜2D to the optical frequency conversion unit 13 and theup signals having frequency λ_(4m) transmitted from the opticalfrequency conversion unit 13 through the optical waveguides 202-21˜2D.The bi-directional multiplexer/demultiplexers or bi-directionalmultiplexer/dividers 510-1˜D can utilize the reverse movement of lightto be realized by supplying input signals from one output of an opticaldemultiplexer or optical divider.

FIG. 6 schematically illustrates a configuration of the opticalfrequency conversion unit 13. The optical frequency conversion unit 13comprises optical frequency conversion circuits 603-01˜0B, 603-11˜R and603-21˜2D for optical frequency converting down signals havingfrequencies λ_(5n), λ_(1n) and λ_(3k) transmitted through the opticalwaveguides 102-01˜0B, 102-11˜1R and 102-21˜2D in the optical frequencydivision multiplexing fashion in accordance with frequency conversioncontrol signals 653-01˜0B, 653-11˜1R and 653-21˜2D to send the convertedsignals onto optical waveguide bundles 103-01˜0B, 103-11˜1R and103-21˜2D, and optical frequency conversion circuit groups 613-11˜1T and613-21˜2D for optical frequency converting up signals having frequencyλ_(op) transmitted through optical waveguide bundles 203-11˜1T and203-21˜2D in the optical frequency division multiplexing fashion inaccordance with frequency conversion control signal 663-11˜1T and663-21˜2D produced from the control unit 11 to send the convertedsignals to optical waveguide bundles 202-11˜1R and 202-21˜2D as signalshaving frequencies λ_(2i) and λ_(4m).

FIG. 10 schematically illustrates a configuration of the opticalfrequency conversion circuit group. The optical frequency conversioncircuit group 613 comprises optical waveguides 215-1˜K, opticalwaveguide bundles 230-1˜K and optical frequency conversion circuits603-1˜K supplied with signals from the optical waveguides 215-1˜K toeffect optical frequency conversion in accordance with frequencyconversion control signals 653-1˜K (which are the same as the controlsignal 663) to send to the optical waveguide bundles 225-1˜K.

FIGS. 11A and 11B schematically illustrate configurations of the opticalfrequency conversion circuit. The optical frequency conversion circuitis supplied with a signal from an optical waveguide 240 and opticalfrequency converts the signal in accordance with frequency conversioncontrol signal 654 to be sent to optical waveguides 251-1˜M (=opticalwaveguide bundle 250). The embodiment employs two kinds of circuitsshown in FIGS. 11A and B. The optical frequency conversion circuit shownin FIG. 11A comprises an optical frequency selector 673 including anoptical demultiplexer 670 and an optical space switch 672, first opticalfrequency conversion elements 605-1˜M for frequency converting inputtedoptical signal, and optical waveguides 241-1˜M for connecting betweenthe optical frequency selector 673 and the optical frequency conversionelements 605-1˜M. The optical frequency selector 673 optical frequencyselects optical signal transmitted through the optical waveguide 240 andsends the selected signal to the optical waveguides 241-1˜M by means fothe optical space switch 672 in accordance with one signal 654-SW of thecontrol signal 654. The selected signal is optical frequency convertedby the optical frequency conversion elements in accordance with thefrequency conversion control signals 654-1˜M. The optical space switch672 is inserted to cause the optical waveguides 251 to correspond to theoptical frequencies, while it can be treated by the terminal interfaceunit 14 depending on system configuration and in this case it isomitted. The optical frequency conversion circuit shown in FIG. 11Bcomprises an optical divider 671, optical frequency selection andconversion elements 605-1˜M for frequency converting inputted opticalsignal and optical waveguides 241-1˜M for connecting the optical divider671 and the optical frequency selection and conversion elements. Theoptical divider 671 distributes optical signal transmitted through theoptical waveguide 240 in optical frequency division multiplexing fashionto the optical frequency selection and conversion elements 605-1˜M to besent to the optical waveguides 241-1˜M. The distributed multiplexedsignals are subjected to optical frequency selection and conversion inthe second optical frequency selection and conversion elements inaccordance with the frequency selection and conversion control signals654-1˜M. Difference between the circuits of FIGS. 11A and 11B is thatthe former must use the complicated optical frequency selector orfrequency fixed optical frequency selector and a main portion of opticalpower supplied to the optical frequency conversion element is couplingloss of optical components and relatively small whereas the latteremploys inexpensive optical components such as optical divider andoptical power supplied to the optical frequency selection and conversionelement is attenuated to one M-th by optical divider. When assignment ofthe frequency is fixed or semi-fixed, the circuit of FIG. 11A is mainlyused, and when assignment of the frequency is dynamic, the circuit ofFIG. 11B is mainly used.

As the optical frequency conversion element, there are known (a) anoptoelectronic integrated circuit having the function that a signal isconverted into an electric signal by a receiver and an optical frequencyvariable light emitting element is used to convert the electric signalinto an optical signal, (b) a frequency shifter in which optical signaland modulation light for frequency to be shifted are added to non-linearoptical material simultaneously, (c) a frequency shifter using apolarizing rotation element, (d) an optical frequency conversion elementhaving an optical filter for converting into an ASK (amplitude shiftkeying) signal and an optical frequency variable laser for convertinginto an FSK (frequency shift keying) signal, and (e) an opticalfrequency conversion element using four-light wave mixture. As theoptical frequency selection and conversion element, there are known (a)an optical frequency conversion element using four-light wave mixtureand (b) an integrated element having a combination of the opticalfrequency conversion element and a variable light filter using a laser.Any of them can be applied to the embodiment, while the opticalfrequency conversion element using four-light wave mixture is actuallyemployed in the embodiment. The optical frequency conversion elementusing the four-light wave mixture has the same configuration as thatdescribed in FIG. 2 of paper by G. Grosskopf, R, Ludwig, H. G. Weber,“140 Mbit/s DPSK Transmission Using An All-Optical Frequency ConverterWith A 400 GHz conversion Range”, Electronics Letters, Vol. 24, No. 17,pp. 1106–1107. According to the paper, a frequency of an input signalSin is shifted by Δf₂ by light emitting sources P1 and P2 having afrequency separated by Δf₁ from that of the input signal Sin. FIG. 12schematically illustrates a configuration thereof. It comprises lightsources 671 and 672, a light amplifier 679, a variable light filter 675,and optical multiplexers 676 and 677. The light sources 671 and 672correspond to lasers P1 and P2 shown in FIG. 2 described in the abovepaper, respectively, and the light amplifier 673 corresponds to thelight amplifier shown in FIG. 2 of the above paper. In the embodiment, afrequency of the light source 671 is set to a frequency (λ₁+Δf₁)separated by Δf₁ from an indication frequency (λ₂) in accordance with aselection indication signal 654-S for indicating a selection frequency,of frequency control signals 654 and a frequency of the light source 672is set to a frequency (λ₂+Δf₁) separated by Δf₁ from an indicationfrequency (λ₂) in accordance with a conversion indication signal 654-Tfor indicating the converted optical frequency. By setting in thismanner, signal having optical frequency λ₁ is shifted by a differencebetween optical frequencies of the lasers 671 and 672. Consequently, theconverted optical frequency becomes a desired optical frequency givenby:λ₁−{(λ₁ +Δf ₁)−(λ₂ +Δf ₁)}=λ₂The optical signal capable of being optical frequency converted in thismanner has a limitation as described in the above-mentioned paper (page1106, left column, fifth line from bottom) and is determined by a lifetime of a carrier of the light amplifier 679 in the embodiment and iswithin about 10 GHz lower than λ₁. The optical frequencies of opticalsignals therein are all shifted. When this operation is utilized, two ormore optical signals can be shifted simultaneously. On the contrary,optical signals having a frequency higher than the frequency disappear.At this time, in order to exactly suppress signals other than desiredoptical frequency, the variable filter is used. In this manner,selection and conversion of optical frequency can be made. Opticalsignals having a plurality of optical frequencies can be selected andconverted simultaneously.

The light sources 671 and 672 adopt (a) a wavelength variable LD or (b)a system in which an optical signal having one optical frequency isselected by primary optical space switch 678 from optical signals havingoptical frequencies λ_(1˜n) distributed through optical waveguides392-1˜n from standard optical source shown in FIG. 13 to be sent tooptical waveguide 391.

FIG. 7 schematically illustrates a configuration of the terminalinterface unit 14. The terminal interface unit 14 comprises interfaces560-1˜U corresponding to the terminal networks 20-01˜U, bi-directionalmultiplexing/demultiplexing portions 571-1˜F corresponding to theterminal networks 20-11˜F, terminal corresponding interfaces 560-1˜F,and a signal connection board 555. Signals transmitted from the terminalnetworks 20-01˜U are divided/demultiplexed in the terminal interfaces560-01˜U if necessary and are distributed to the optical waveguidebundles 103-ij (where ij=01˜B, 11˜R, and 21˜D) while signals transmittedfrom the terminal networks 20-11˜F are demultiplexed by thebi-directional multiplexing/demultiplexing portions 571-1F and are thendivided/demultiplexed in the terminal interfaces 560-11˜F if necessaryto be distributed to the optical waveguide bundles 103-ij (ij=01˜B,11˜R, and 21˜D). Signals from the optical waveguide bundles 201-ij(ij=11˜T and 21˜D) are distributed to the terminal correspondinginterfaces 560-01˜U 560-11˜F in the signal connection board 555 andmultiplexed if necessary to be transmitted through the opticalwaveguides 300-01˜U to the terminal networks 20-01˜U, while signals ofoptical waveguides 300-11˜F are multiplexed by the bi-directionalmultiplexing/demultiplexing portions 571-1˜F and transmitted throughoptical waveguides 340-11˜F to the terminal networks 20-11˜F. Theoptical signal distribution and collection portion 555 re-assemblessignals from the optical frequency conversion unit 13 in correspondingmanner to the terminal networks and distributes the signals to theterminal interfaces 560-01˜U and 560-11˜F. Further, optical signals fromthe terminal interfaces 560-01˜U and 560-11˜F are distributed to opticalwaveguides designated by the optical frequency conversion unit 13.

FIG. 14 schematically illustrates a configuration of the terminalinterface 560. The terminal interface 560 comprises an opticaldistributor 681 constituted by an optical divider or an opticaldemultiplexer, a space switch 680, an optical multiplexer 682 andoptical waveguides 270-1˜Q. Signal having optical frequency {λ_(ov)}transmitted through optical fiber 300 from terminal is distributed topredetermined optical frequency by the optical distributor 681 and issent to the optical space switch 680 through the optical waveguides270-1˜Q. The optical space switch 680 distributes the signal suppliedthrough the optical waveguides 270-1˜Q to the optical waveguide bundle321 in accordance with control signal 690 produced from the control unit11. On the contrary, signals transmitted through the optical waveguidebundle 320 are multiplexed by the optical multiplexer 682 and are sentthrough the fiber 300 to the terminal network or the bi-directionalmultiplexing/demultiplexing portion 571. However, when Q is 1, there isa case where the optical demultiplexer 681 and the space switch 680 areomitted and the space switch 680 is composed of a mere optical waveguidewiring. Further, there is a case where one or more second opticalmultiplexers are connected between the optical distributor 681 and thespace switch 680 depending on assignment of optical frequency tomultiplex signals distributed by the optical distributor 681. Inaddition, there is a case where one or more optical frequency filtersare connected between the optical distributor 681 and the space switch680 to send only part of signals distributed by the optical distributor681 to the space switch 680.

FIG. 15 schematically illustrates a configuration of the optical signaldistribution and collection portion 555. The optical signal distributionand collection portion 555 comprises an optical frequency converterinterface 685, an optical space switch 686, a terminal interface 687,and optical waveguide bundles 275, 276, 277, 278, 279 and 280. Theoptical frequency converter interface 685 distributes signals to whichcircuits or lines are set by the optical space switch 686, of signalsfrom the optical frequency conversion unit 13 and fixed lines to theoptical waveguide bundles 275 and 277, respectively, whereasre-assembles signals supplied through the optical waveguide bundles 276and 278 in corresponding manner to the optical frequency conversion unit13. The terminal terminator interface 687 distributes signals to whichcircuits or lines are set by the optical space switch 686, of signalsfrom the terminal interface 560 and fixed lines to the optical waveguidebundles 280 and 276, respectively, whereas the interface 687re-assembles signals supplied through the optical waveguide bundles 275and 279 in corresponding manner to the terminal interface. The opticalfrequency converter interface 685 and the terminal correspondingterminator interface 687 have the same configuration and include acombination circuit of an optical distributor having an optical divideror an optical demultiplexer, an optical multiplexer, an opticalwaveguide wiring and an optical frequency filter.

FIGS. 8A, 8B and 9 schematically illustrate the terminal network 20.FIGS. 8A and 8B illustrate a terminal network including two wire opticalfiber cables each transmitting up and down signals, respectively, andFIG. 9 illustrates a terminal network including a single optical fibercable for transmitting up and down signals in optical frequency divisionmultiplexing fashion.

FIG. 8A illustrates a configuration in which one or a plurality of firstterminal nodes 22-1˜q and a terminating portion 21 are connected inseries through two fibers 300, 322-1˜q, 400 and 422-1˜q, and FIG. 8Billustrates a configuration in which one or a plurality of secondterminal nodes 23-1˜p are connected in open loop through fibers 300 and322-1˜p (322-p=400). In FIG. 8A, there is a case where the terminatingportion 21 is integrated into the terminal node 22-q.

FIG. 9 illustrates a configuration in which one or a plurality of thirdterminal nodes 25-1˜r and a terminating portion 24 are connected inseries through single fiber 340 and 345-1˜r. There is a case where theterminating portion 24 is integrated into the terminal node 25-r.

FIGS. 16A and 16B schematically illustrate configurations of the firstterminal node 22. The first terminal node of FIG. 16A comprises fibers322 and 422 connected to the node, that is, the remote node or aterminal node which is connected nearer to the node and adjacent to thisfirst terminal node, fibers 322′ and 422′ connected to a next terminalnode, a terminal 30 connected through two fibers 372 and 472 to transmitup and down signals, a node optical frequency demultiplexer 690 and anode optical frequency multiplexer 691. Signal transmitted through theoptical fiber 322 is demultiplexed or divided or optical frequencyconverted or optical frequency selected/converted if necessary by theoptical frequency demultiplexer 690 to be transmitted through theoptical fiber 372 to the terminal 30. The remaining optical signals ofthe optical frequency demultiplexer 690 are transmitted through theoptical fiber 322′ to the next terminal node as they are. Signaltransmitted through the optical fiber 472 from the terminal 30 isoptical wavelength converted by the node optical frequency multiplexer691 if necessary and is multiplexed with signals from the optical fiber422′ to be transmitted to the optical fiber 422. The first terminal node22 shown in FIG. 16B comprises fibers 322 and 422 connected to the nodeor a terminal node which is connected nearer to the node and adjacent tothis first terminal node, fibers 322′ and 422′ connected to a nextterminal node, a terminal 30 connected through a fiber 482 to transmitup and down signals in optical frequency division multiplexing fashion,a node optical frequency demultiplexer 690, a node optical frequencymultiplexer 691 and an optical multiplexer/demultiplexer 692. Signaltransmitted through the optical fiber 322 is demultiplexed or divided oroptical frequency converted or optical frequency selected/converted ifnecessary by the optical frequency demultiplexer 690 and is opticalfrequency multiplexed by the optical multiplexer/ demultiplexer 692 tobe transmitted through the optical fiber 472 to the terminal 30. Theremaining optical signals of the optical frequency demultiplexer 690 aretransmitted through the optical fiber 322′ to the next terminal node asthey are. Signal transmitted through the optical fiber 482 from theterminal 30 is demultiplexed by the optical multiplexer/demultiplexer692 and is optical wavelength converted by the node optical frequencymultiplexer 691 if necessary and is multiplexed with signals from theoptical fiber 422′ to be transmitted to the optical fiber 422.

FIGS. 17A and 17B schematically illustrate configurations of the secondterminal node 23. The second terminal node 23 shown in FIG. 17Acomprises a fiber 322 connected to the node, that is, the remote node ora terminal node which is connected nearer to the node and adjacent tothis second terminal node, a fiber 322′ connected to a next terminalnode, a terminal 30 connected through two fibers 372 and 472 to transmitup and down signals, a node optical frequency demultiplexer 690, a nodeoptical frequency multiplexer 691 and an optical multiplexer 695.Signals transmitted through the optical fiber 322 are opticallydemultiplexed or divided or if necessary optical frequency converted oroptical frequency selected/converted by the optical frequencydemultiplexer 690 to be transmitted to the terminal 30 through theoptical fiber 372. Signals transmitted through the optical fiber 472from the terminal 30 are optical wavelength converted by the nodeoptical frequency multiplexer 691 if necessary and are multiplexed withoptical signals from the optical frequency demultiplexer 690 by theoptical multiplexed 695 to be sent to the optical fiber 322′. The secondterminal node 23 shown in FIG. 17B comprises a fiber 322 connected tothe node, that is, the remote node or a terminal node which is connectednearer to the node and adjacent to this second terminal node, a fiber322′ connected to a next terminal node, a terminal 30 connected throughfiber 482 to transmit up and down signals in optical frequency divisionmultiplexing fashion, a node optical frequency demultiplexer 690, a nodeoptical frequency multiplexer 691, an optical multiplexer/demultiplexer692 and an optical multiplexer 695. Signals transmitted through theoptical fiber 322 are optically demultiplexed or divided or if necessaryoptical frequency converted or optical frequency selected/converted bythe optical frequency demultiplexer 690 and are optical frequencymultiplexed by the optical multiplexer/demultiplexer 692 to betransmitted to the terminal 30 through the optical fiber 482. Signalstransmitted through the optical fiber 482 from the terminal 30 isdemultiplexed by the optical multiplexer/demultiplexer 692, are opticalwavelength converted by the node optical frequency multiplexer 691 ifnecessary and are then multiplexed with the optical signals from theoptical frequency demultiplexer 690 by the optical multiplexer 695 to besent to the optical fiber 322′.

FIGS. 18A and 18B schematically illustrate configurations of the thirdterminal node 24. The third terminal node 24 of FIG. 18A comprises afiber 322 connected to the node, that is, the remote node or a terminalnode which is connected nearer to the node and adjacent to this thirdterminal node, a fiber 322′ connected to a next terminal node, aterminal 30 connected through two fibers 372 and 472 to transmit up anddown signal, a node optical frequency demultiplexer 690, a node opticalfrequency multiplexer 691 and optical multiplexer/demultiplexer 693 and694. The optical multiplexer/demultiplexer 693 demultiplexes transmittedsignals of bi-directional signals on the fiber 322 to be sent to thenode optical frequency demultiplexer 690 and multiplexes signals fromthe node optical frequency multiplexer 691 to be sent to the fiber 322as bi-directional signals. The optical multiplexer/demultiplexer 694multiplexes signals from the node optical frequency demultiplexer 690 tobe sent to the fiber 322′ as bi-directional signals and demultiplexestransmitted signals of bi-directional signals on the fiber 322′ to besent to the node optical frequency multiplexer 691. Signals transmittedthrough the optical fiber 322 are optically demultiplexed or divided orif necessary optical frequency converted or optical frequencyselected/converted by the node optical frequency demultiplexer 690 to betransmitted through the optical fiber 372 to the terminal 30. Theremaining signals of the node optical frequency demultiplexer 690 issent to the optical multiplexer/demultiplexer 694. Signals transmittedthrough the optical fiber 472 from the terminal 30 are opticalwavelength converted by the node optical frequency multiplexer 691 ifnecessary and are multiplexed with signals from the opticalmultiplexer/demultiplexer 694 to be sent to the opticalmultiplexer/demultiplexer 693. The third terminal node 23 shown in FIG.18B comprises fibers 322 connected to the node, that is, the remote nodeor a terminal node which is connected nearer to the node and adjacent tothis third terminal node, a fiber 322; connected to a next terminalnode, a terminal 30 connected through fiber 482 to transmit up and downsignals in optical frequency division multiplexing fashion, a nodeoptical frequency demultiplexer 690, a node optical frequencymultiplexer 691, an optical multiplexer/demultiplexer 692 and opticalmultiplexers/demultiplexers 693 and 694. The opticalmultiplexers/demultiplexers 693 and 694 have the same function as thatof the optical multiplexers/demultiplexers 693 and 694. Signalstransmitted through the optical fiber 322 are optically demultiplexed ordivided or if necessary optical frequency converted or optical frequencyselected/converted by the optical frequency demultiplexer 690 and areoptical frequency multiplexed by the optical multiplexer/demultiplexer692 to be transmitted to the terminal 30 through the optical fiber 482.Signals transmitted through the optical fiber 482 from the terminal 30is demultiplexed by the optical multiplexer/demultiplexer 692, areoptical wavelength converted by the node optical frequency multiplexer691 if necessary and are then multiplexed with the optical signals fromthe optical frequency demultiplexer 690 by the optical multiplexer 695to be sent to the optical fiber 322′.

FIGS. 19A to 19C schematically illustrate configurations of the nodeoptical frequency demultiplexer 690. As the node optical frequencydemultiplexer 690, one of three kinds of configurations shown in FIGS.19A, B and C or a combination thereof is employed in accordance with thepresence of reception of broadcasting signal or receivable opticalfrequency of terminal or cost. The node optical frequency demultiplexershown in FIG. 19A includes an optical demultiplexer or optical divider590. Optical signal is demultiplexed by the optical demultiplexer oroptical divider 590 to be sent to the terminal. The node opticalfrequency demultiplexer shown in FIG. 19B includes an opticaldemultiplexer 591 and an optical frequency conversion element 592.Optical signal selected and demultiplexed by the optical demultiplexer591 is optical frequency converted by the optical frequency conversionelement 592 to be sent to the terminal. The node optical frequencydemultiplexer 690 shown in FIG. 19B includes an optical demultiplexer oroptical divider 590 and an optical frequency selection/conversionelement 593. Optical signal demultiplexed by the optical demultiplexeror optical divider 590 is optical frequency selected/converted by theoptical frequency selection/conversion element 593 to be sent to theterminal.

FIG. 20 schematically illustrates a configuration of the node opticalfrequency multiplexer 691. The node optical frequency multiplexer 691comprises an optical multiplexer 594 and an optical frequency conversionelement 595. Signals from the terminal are optical frequency convertedby the optical frequency conversion element 595 and are multiplexed bythe optical multiplexed 594. There is a case where the optical frequencyconversion element 595 is omitted depending on a cost and signal opticalfrequency of terminal.

In the embodiment, the terminal node is provided in each terminal inorder to increase the reliability between terminals, while a pluralityof terminals can be connected to one terminal node as in the prior art.There are star, loop and ring connections.

The optical demultiplexer, optical divider, optical multiplexer andoptical multiplexer/demultiplexer are known technique to those skilledin the prior art and are used heretofore and in other transmissionapparatuses or the like.

In the embodiment, with signals which do not require the opticalfrequency conversion, the optical frequency conversion can be omitted inthe optical frequency conversion unit 13 or the terminal nodes 21 to 25.

In the embodiment, the bundle includes a single wire or line orwaveguide in accordance with a network scale or configuration.

According to the present invention, since the signal from the upper nodeis optical frequency selected and converted to the optical frequencyassigned to each terminal to which the signal is to be transmitted inthe node, the privacy between the terminal networks is ensured.

Further, since the signal from the terminal network is optical frequencyconverted in the node to be sent to the upper node, a failure in theterminal network does not influence the whole system and accordingly thenetwork system with high reliability can be attained.

In addition, since the optical frequency of the terminal and the opticalfrequency of the signal between the node and the upper node are assignedindependently and dynamically, the network with high reliability andflexibility can be attained.

The signals between the node and the upper node can be multiplexed inextremely high density by the coherent technique and a large capacity ofinformation can be exchanged.

Further, since the optical frequency of the signals to be transmittedand received of the terminal is optical frequency converted in theterminal node and the node, the optical frequency can be common betweenthe terminals. The same transmission and reception optical frequency canbe used in the whole terminals. Thus, the optical frequency tuning inthe terminal is unnecessary or simple, so that operability of theterminal is satisfactory and movement and replacement of the terminalare easy and the cost of the terminal is inexpensive.

Since the optical frequency is assigned flexibly, the form of theterminal network and the degree of freedom in the transmission systemare wide.

1. A transmission apparatus connected to an optical signal transmissionline, first network and second network, comprising: an optical frequencyselection unit for selecting a first optical signal of a first opticalfrequency and a second optical signal of a second optical frequencyamong an optical frequency division multiplexed signal received fromsaid optical signal transmission line and transmitting said selectedfirst and second optical signals separately, a first optical frequencyconversion unit connected to said optical frequency selection unit andsaid first network for converting a frequency of said first opticalsignal from said first optical frequency to a third optical frequencyand transmitting an optical signal of said third optical frequencytoward said first network, a second optical frequency conversion unitconnected to said optical frequency selection unit and said secondnetwork for converting a frequency of said second optical signal fromsaid second optical frequency to said third optical frequency andtransmitting an optical signal of said third optical frequency towardsaid second network, wherein said first optical frequency is allottedfor transmissions of signals from said optical transmission line to saidfirst network, said second optical frequency is allotted fortransmissions of signals from said optical transmission line to saidsecond network, and said third optical frequency is allotted fortransmissions of signals from said transmission apparatus to said firstand second networks.
 2. A transmission apparatus according to claim 1,comprising a control unit for instructing said optical frequencyselection unit to transmit said first optical signal to said firstoptical frequency conversion unit and to transmit said second opticalsignal to said second optical frequency conversion unit.
 3. Atransmission apparatus according to claim 2, wherein said control unitis notified via said optical transmission line that said first opticalfrequency corresponds to said first network and that said second opticalfrequency corresponds to said second network.
 4. A transmissionapparatus according to claim 1, comprising a control unit for indicatingsaid first and second optical frequency conversion unit to convert afrequency of input optical signal to said third frequency that isallotted to said first apparatus and second apparatus.
 5. A transmissionapparatus according to claim 1, wherein said optical frequency selectionunit comprising: a demultiplexer for demultiplexing said opticalfrequency division multiplexed signal and transmitting said firstoptical signal and said second optical signal separately, and an opticalspace switch connected to said demultiplexer for guiding said firstoptical signal and said second optical signal to said first opticalfrequency conversion unit and to said second optical frequencyconversion unit respectively based upon instructions of said controller.6. A transmission apparatus according to claim 1, further connected tothe other optical transmission line and further comprising: a thirdoptical frequency conversion unit connected to said first network forreceiving a third optical signal of fourth optical frequency from saidfirst network and converting a frequency of said third optical signalfrom said fourth optical frequency to a fifth optical frequency andtransmitting said converted third optical signal toward said otheroptical transmission line, a fourth optical frequency conversion unitconnected to said second network for receiving a fourth optical signalof fourth optical frequency from said second network and converting afrequency of said fourth optical signal from said fourth opticalfrequency to a sixth optical frequency and transmitting said convertedfourth optical signal toward said other optical transmission line,wherein said fourth optical frequency Is allotted for transmissions ofsignals from said first and second networks to said transmissionapparatus, said fifth optical frequency Is allotted for transmissions ofsignals from said first network to said other optical transmission line,said sixth optical frequency is allotted for transmissions of signalsfrom said second network to mid other optical transmission line.
 7. Atransmission apparatus connected to an optical signal transmission lineand further connected to first and second networks, comprising: anoptical divider for dividing an optical frequency division multiplexedsignal received from said optical signal transmission line Into a firstoptical frequency division multiplexed signal and a second opticalfrequency division multiplexed signal, a first optical frequencyselection unit receiving said first optical frequency divisionmultiplexed signal for selecting a first optical signal of a firstoptical frequency among said first optical frequency divisionmultiplexed signal and transmitting said selected first optical signal,a second optical frequency selection unit receiving said second opticalfrequency division multiplexed signal for selecting a second opticalsignal of a second optical frequency among said second optical frequencydivision multiplexed signal and transmitting said selected secondoptical signal, a first optical frequency conversion unit connected tosaid first optical frequency selection unit and said first network forconverting a frequency of said first optical signal from said firstoptical frequency to a third optical frequency and transmitting anoptical signal of said third optical frequency toward said firstnetwork, and a second optical frequency conversion unit connected tosaid second optical frequency selection unit and said second network forconverting a frequency of said second optical signal from said secondoptical frequency to said third optical frequency and transmitting anoptical signal of said third optical frequency toward said secondnetwork.
 8. A transmission apparatus according to claim 7, wherein saidfirst optical frequency is allotted for transmissions of signals fromsaid optical transmission line to said first network, said secondoptical frequency is allotted for transmissions of signals from saidoptical transmission line to said second network, and said third opticalfrequency is allotted for transmissions of signals from saidtransmission apparatus to said first and second networks.
 9. Atransmission apparatus according to claim 7, comprising a control unitfor indicating said first and second optical frequency conversion unitto convert a frequency of input optical signal to said third frequencythat is allotted to said first apparatus and second apparatus.
 10. Atransmission apparatus according to claim 7, further connected to theother optical transmission line and further comprising: a third opticalfrequency conversion unit connected to said first network for receivinga third optical signal of fourth optical frequency from said firstnetwork and converting a frequency of said third optical signal fromsaid fourth optical frequency to a fifth optical frequency andtransmitting said converted third optical signal toward said otheroptical transmission line, a fourth optical frequency conversion unitconnected to said second network for receiving a fourth optical signalof fourth optical frequency from said second network and converting afrequency of said fourth optical signal from said fourth opticalfrequency to a sixth optical frequency and transmitting said convertedfourth optical signal toward said other optical transmission line,wherein said fourth optical frequency is allotted for transmissions ofsignals from said first and second networks to said transmissionapparatus, said fifth optical frequency is allotted for transmissions ofsignals from said first network to said other optical transmission line,said sixth optical frequency is allotted for transmissions of signalsfrom said second network to said other optical transmission line.